1. Field of the invention
The invention relates to a lockup clutch for a torque converter.
2. Description of the Related Art
FIG. 14 shows a structure of a torque converter 30 with a lockup clutch. In this drawing, numeral 2 represents a front cover connected directly with a shaft of a machine; 23, a pump case fixed to the front cover 2; 3, a pump; 4, a turbine; 24, a turbine case; 25, a stator; and 28, an output shaft connected directly with the turbine case 24.
The torque converter is a fluid coupling which is a combination of the pump and the turbine. Slip exists even if the rotation of the pump substantially coincides with the turbine. Accordingly, the transmission efficiency of the torque converter is lower than that of a directly connected mechanical clutch.
Therefore, a torque converter with a lockup clutch, a mechanical clutch has been utilized. In FIG. 14, numeral 1 represents a piston; 26, 27, springs for a damper; 34, a driven disk; 29, a direct-connected part with the turbine case 24; 31, a first oil chamber; 32, a second oil chamber; 40, a friction member bonded on the piston 1; and 41, an oil groove formed on the piston 1. As a pressure of the second oil chamber 32 is made higher, the piston 1 is compressedly contacted to the front cover 2 through the friction member 40, and the piston 1 rotates together with the front cover 2. The rotation is transmitted to the driven disk 34 through a connecting part 33 of a spline or the like, and then the rotation is transmitted to the output shaft 28, by the direct-connected part 29.
In such a lockup clutch, high pressure oil from the second oil chamber 32 penetrates into the friction member during operation. Consequently, the high pressure oil becomes a reaction force opposing a force which presses the piston 1, thereby decreasing torque transmission capacity. Therefore, for example, as shown in FIG. 14, the oil groove 41 is formed on the piston 1 so that the high pressure oil penetrating into the friction member during operation immediately flows into the first oil chamber 31 from the oil groove 41.
In the conventional clutch mechanism, since the oil groove is formed directly on a press surface of the clutch, namely either, the friction member, the front cover, the piston or the like, the strength of the part on which the oil groove is located decreases.
In the processing of the oil groove, cutting work that must be done on a hard material to form the groove is difficult. Also, cutting has the problem of increasing the production cost because workability of the hard material is so bad that it takes a long time to complete. When the oil groove is formed by molding, the life of the die is short because the die is narrow. On the other hand, when the oil groove is formed by cutting, the material into which the groove is cut is weakened thus and the life of the part is reduced. In addition, since the piston is heat treated during molding, cutting it thereafter to form the oil groove lengthens the manufacturing time required. Moreover, the piston to which quenching treatment is applied has a very hard surface where the oil groove formed. Accordingly such a piston is should not be cut to form the oil groove.